The physical shipment of materials, including the shipment of mail, merchandise, raw materials, and other goods, is an integral part of any economy. Typically, the materials are shipped in a type of shipping containment or cargo box. Such containments or boxes include semi-trailers, large trucks, and rail cars as well as inter-modal containers that can be carried on container ships or cargo planes. However, such shipping or cargo containers can also be used for illegal transportation of contraband. Detection of these threats requires a safe and accurate inspection system, yet one that is also highly efficient so as to not impose an excessive delay or processing burden on very high volumes of goods.
In particular, there exists a requirement for automated scanning of cargo carried by train for the purpose of security inspection. This cargo is typically in containerized form, whereby the container has a standard size and shape. Trains carrying cargo containers typically travel at a high speed (in the range 20 km/h to 150 km/h); therefore, the security inspection process should be capable of being conducted at these high speeds without interfering with the flow of trade. Further, the system must not expose any worker who may be present on the cargo train to radiation, such as a driver or guard, while radiation exposure to an individual who may be hidden within the cargo should be reduced to a reasonably low level.
Known scanning processes for inspection of containerized cargo include X-ray scanning, chemical analysis of vapour emitting from the cargo, listening to sound from the cargo to detect living objects and eventually interventional manual search of the cargo by one or more security officials.
Most common in almost all regions of the world is the use of X-ray scanning for scanning containerized cargo. Here, a variety of systems have been developed including mobile scanners (which drive past the object under investigation during scanning), trailer-based scanners (where the vehicle under inspection drives through the inspection zone), gantry-based scanners (which drive along rails past the object under inspection during a scan) and portal mode scanners (where a vehicle drives through the scanner and either the entire vehicle or just the cargo is scanned).
Most X-ray scanning systems use either a linear accelerator or an X-ray tube to produce the necessary penetrating radiation within a tightly collimated fan-beam of X-rays. Linear accelerator systems use energies typically in the range 1 MV to 9 MV while X-ray tube based systems use energies in the range 100 kVp to 450 kVp. Alternate radiation sources include gamma-ray emitting materials such as Co-60 or Cs-137 or X-ray sources such as betatrons. Occasionally, neutron sources are used for scanning cargo, including isotopic sources such as Am-Be or Cf-252 or electronic source such as D-D or D-T pulsed neutron generators.
In each case, the scanning speed of the system is constrained by factors such as the pulse rate achievable from the Linear Accelerator or the allowable dose rate from the X-ray tube to provide sufficient penetration of the cargo while simultaneously providing safe scanning of occupied cargo.
Systems known to those of skill in the art operate at scanning speeds of less than 15 km/h to preserve an acceptable image quality. This speed is determined by the pulse rate from the fan-beam X-ray source and the relatively narrow width of the X-ray sensor array.
There is therefore a need for a scanning system which is not constrained in providing higher scanning speeds that are required for screening cargo on rail cars. Additionally, such a scanning system is required to maintain excellent image quality regardless of scanning speed.